Exemplary embodiments of the present invention relate to a combustion process with auto-ignition for direct-injection internal combustion engines and to an internal combustion engine, in particular for a motor vehicle, for carrying out such a combustion process.
German patent document DE 10 2006 020 642 A1 discloses an auto-ignition internal combustion engine for implementing a combustion process in which the internal combustion engine has at least one cylinder whose combustion chamber is laterally delimited by a cylinder wall, and is delimited axially on the one hand by a cylinder head, and axially on the other hand by an adjustable-stroke piston in the cylinder. The cylinder wall defines a longitudinal center axis of the cylinder. In addition, the internal combustion engine includes one injection nozzle per cylinder which is situated coaxially with respect to the combustion chamber, i.e., situated in the cylinder head essentially coaxially with respect to the longitudinal center axis. The particular piston is designed as a stepped piston having an annular circumferential piston step that is axially recessed in the piston relative to an annular piston crown, and which via an annular circumferential jet divider contour merges into a piston cavity which is axially recessed in the piston relative to the piston step. The particular injection nozzle is designed in such a way that for a combustion operation it is able to simultaneously inject multiple injection jets into the combustion chamber in a star-shaped pattern. In the known combustion process, the injection jets are each divided at the jet divider contour into a first partial quantity that essentially enters into the piston cavity, and a second partial quantity that spreads out essentially over the piston crown via the piston step in the direction of the cylinder wall and the cylinder head. A first combustion front and a second combustion front are formed by the first partial quantity and the second partial quantity, respectively. In addition, after striking the jet divider contour in the piston step, the respective injection jets are each divided into third partial quantities which, starting from the particular injection jet, spread out essentially on both sides in the peripheral direction in opposite directions along the piston step. In the further course of the combustion process, the respective third partial quantities collide with one another in each case in the peripheral direction between two adjacent injection jets within the piston step and are deflected radially inwardly. The formation and guiding of these third partial quantities are brought about by the injection jets striking the jet divider contour and the piston step. Due to the third partial quantities moving away from the piston step, a third combustion front is formed which essentially spreads out radially into a gap formed in the peripheral direction between adjacent injection jets. This type of combustion process may also be referred to as a “triple front combustion process,” since essentially three combustion fronts or flame fronts in each case spatially spread out in the combustion chamber for each injection jet.
Since the combustion fronts make effective use of the available space in the combustion chamber, the known combustion process utilizes to a great extent the fresh gas or fresh gas-exhaust gas mixture that is available in the combustion chamber, which significantly reduces the soot formation during the combustion. In addition, due to the combustion of the third combustion front in a time-delayed manner compared to the first two combustion fronts, local peak temperatures are lowered, thus reducing NOx formation.
For forming the third combustion front, the third partial quantities of adjacent injection jets in the piston step collide with one another with high momentum. The momentum of the third partial quantities is formed from the product of the mass of the third partial quantities and their velocity. If the momentum of the third partial quantities is sufficiently high, when the third partial quantities collide with one another in the piston step they are deflected into the combustion chamber, starting from the space that is spanned or opened by the piston step. The respective third partial quantities that are deflected together form a third combustion front oriented essentially in the direction of the longitudinal center axis between the respective injection jets. A high rate of turbulence is advantageously generated in the combustion chamber by means of the third combustion front, so that the compatibility with exhaust gas recirculated into the combustion chamber increases, as the result of which the NOx emissions may be further reduced by means of the increased EGR portion in the combustion chamber. However, in the known combustion process, only a small time window or only a small crank angle is available for dividing the injection jets at the jet divider contour into optimally dimensioned three partial quantities, so that optimal conditions for forming the third combustion front are present in only a few load and speed ranges, as the result of which the soot emissions in particular cannot be reduced to the desired degree compared to conventional diesel combustion.
Exemplary embodiments of the present invention are directed to an improved auto-ignition combustion process and an internal combustion engine which is suitable for this purpose, characterized by an improved division of the introduced fuel into the three partial quantities. For downwardly moving pistons, it is thus possible to maintain the combustion process for a longer period of time than heretofore.
The invention is based on the general concept of deflecting the injection jets in the direction of the piston in an area situated upstream from the jet divider contour. As a result of this measure, during the beginning expansion stroke the injection jets may be deflected toward the piston, which is moving away from the cylinder head. Consequently, the injection jets may continue to strike the jet divider contour or the piston in the area of the jet divider contour, even when the piston is moving away from the cylinder head, in order to still divide the particular injection jet into three partial quantities. As a result, a longer time period or an increased crankshaft angle is available for the optimal division of the particular injection jets. By varying the injection duration, injection pressure, and injection timing, in particular at the start of injection, third partial quantities having a sufficiently large fuel mass and fuel velocity, and thus a sufficiently high inherent momentum of the third partial quantities, may be reproducibly generated over an extended characteristic map range, so that the third combustion front may form in a gap between two adjacent injection jets and thus make optimal use of spatially distributed combustion air or a combustion air-exhaust gas in the combustion chamber, as the result of which the soot emissions in particular may be reduced. In addition, by deflection of the injection jet according to the invention, the wetting of the cylinder wall with fuel from the second partial quantity is at least greatly reduced, since the injection jet essentially strikes the jet divider contour or adjoining areas of the piston and does not spread over the piston crown or directly strike the cylinder wall. In addition, by means of the piston step, the third partial quantities are separated from the respective injection jets, so that less fuel is available for the second partial quantity, as the result of which the momentum inherent to the second partial quantity becomes lower, and therefore the penetration depth of the second partial quantity is reduced. In addition, the axial deflection of the second partial quantity in the piston step toward the cylinder head results in damming of the second partial quantity between the cylinder head and the piston, so that the fuel flowing into the second combustion front due to the injection is decreased, thus decelerating the radial spreading of the second partial quantity or the second combustion front. By means of these two effects, contact of the second combustion front with the relatively cold cylinder or cylinder wall may advantageously be minimized, thus greatly decreasing the undesirable heat transfer between the second combustion front and the cylinder wall, so that soot formation from the second combustion front at the relatively cold cylinder wall may be reduced. In addition, washout of oil from the cylinder wall and infiltration of soot into the engine oil may be decreased.
According to the present invention, by means of a squish gap flow, a swirl flow, and a jet flow, a resultant flow is generated whose flow vector includes at least one essential portion that forms above the piston cavity, coaxially with respect to the longitudinal center axis of the cylinder in the direction of the piston cavity. This forms the squish gap flow between the piston crown and the cylinder head, which is oriented essentially from the cylinder wall toward the center of the cylinder. The swirl flow, i.e., the swirl of the combustion air, is directed essentially about the longitudinal center axis of the cylinder. The jet flow is formed in the course of the injection jet that spreads out in the combustion chamber, the combustion air that surrounds the injection jet being entrained in the direction of the injection jet. The resultant flow strikes the injection jets above the piston cavity, shifted radially inwardly with respect to the jet divider contour, i.e., upstream from the jet divider contour, and deflects in the direction of the piston.
According to one advantageous embodiment, the injection jets are injected so that they first strike the jet divider contour off-center, namely, offset toward the piston cavity, in the course of the injection spread over the center of the jet divider contour, and lastly, strike the jet divider contour offset toward the piston step. In other words, the propagating injection jets do not first strike the jet divider contour, offset on the jet divider contour toward the piston step, but, rather, strike the jet divider contour only in the further course of the injection, so that the injected quantity of fuel may be optimally divided into a first partial quantity, a second partial quantity, and third partial quantities, as the result of which the third partial quantities in particular acquire sufficiently high momentum and are able to form as the third combustion front when they collide. This does not change with the expansion stroke of the piston, which moves away from the cylinder head; instead, the deflection of the injection jets according to the invention in the direction of the piston essentially off-center, offset with respect to the piston step, is maintained, as the result of which the largest possible fuel mass enters into the piston step at high velocity, and in third partial quantities spreads out in the piston step in the peripheral direction.
According to one advantageous embodiment, the injection jets are injected in such a way that they initially strike the jet divider contour off-center, namely, offset toward the piston cavity, and in the course of the injection subsequently strike the jet divider contour when the piston is moving downwardly. This applies for injection quantities that have short injection durations as a function of the injection pressure, so that the injection jet strikes the jet divider contour at the end of its injection duration. The resultant flow according to the invention to the injection jet also advantageously acts for smaller injection quantities, as the result of which the injected quantity of fuel may still be optimally divided into a first partial quantity, a second partial quantity, and third partial quantities.
According to another advantageous embodiment, by means of the respective injection nozzle 7 to 12, preferably 10 to 12, in particular 10, injection jets may be generated at the same time. The mentioned number of injection jets results in a small angle between the injection jets, so that the adjacent step jets that are moving toward one another at the piston step must cover comparatively short distances in the piston step, so that the third partial quantities lose little velocity and collide with a particularly high momentum, and the third combustion front may be formed in a particularly effective manner.
In one particularly advantageous embodiment, the particular injection jet is introduced during an injection duration of 40 crankshaft angle degrees maximum. Particularly stable combustion may thus be achieved, since the three above-mentioned flame fronts may thus be implemented in a particularly reliable manner.
According to another advantageous embodiment, the injection jets may be injected at a spray cone angle in an angular range of, for example, approximately 140°to approximately 160°, preferably approximately 152°.
It is particularly advantageous when the spray cone angle spreads out essentially coaxially with respect to the longitudinal center axis of the cylinder, so that the injection jets in each case strike at the same height of the jet divider contour, as the result of which the third combustion fronts may uniformly spread out toward one another in the combustion chamber.
In another advantageous embodiment, the injection jets are introduced into the combustion chamber at an injection pressure of 500 bar to 2800 bar. The injection jets advantageously have a high velocity during spraying into the combustion chamber, so that the injection jets have a high inherent momentum, thus simplifying formation of the third combustion front.
In one particularly advantageous design, the first partial quantities are deflected in an undercut in the piston cavity. The jet divider contour thus protrudes radially inwardly beyond an outer wall of the piston cavity. The first partial quantities, i.e., the first combustion front of the respective injection jets, flow over the undercut in the direction of the center of the piston, starting from the jet divider contour. Due to the improved deflection essentially tangentially with respect to the undercut in the direction of the longitudinal center axis, the first partial quantities advantageously exit at the transition of the undercut into the piston cavity base or a piston cone, resulting in an improved distribution of the first partial quantities in the combustion air in the piston cavity, as the result of which the effectiveness of the combustion may be increased, and the emissions may be further reduced.
In one particular design of the undercut, the undercut is up to 3% of a piston diameter.
The swirl flow may be generated within the combustion air in the combustion chamber by means of an appropriate configuration and/or orientation of the intake openings and/or the intake ducts, and/or by appropriate contouring in the area of the valve seats of the intake valves. It is also known to generate swirl by means of swirl flaps in the intake duct, or a combination of swirl flaps and a device mentioned above. By means of the swirl of the combustion air, a resultant flow may be formed that deflects the injection jets in the direction of the piston. In addition, in particular the intermixing of combustion air and recirculated exhaust gas is enhanced, since the charge in the cylinder is moved about the longitudinal center axis by means of the swirl flow.
For generating a charge, acted on by swirl, of fresh air or fresh air and recirculated exhaust gas in the combustion chamber after a charge exchange, the internal combustion engine may be equipped with suitable swirl generation means. Examples of such swirl generation means include the measures mentioned above.
The swirl in the charge may be generated with an i-theta of up to 4.5 maximum. In particular for utility vehicle engines, a comparatively low swirl of 0.3 to 2.5 may be sufficient to generate the desired resultant flow. For smaller-displacement engines, a higher swirl of up to 4.5 may be practical.
In one particular embodiment the swirl may be changeable. In this way the swirl may be advantageously adapted to the operating conditions, so that further degrees of freedom with regard to the deflection of the injection jets may be obtained, as the result of which the deflection of the injection jets according to the invention may be used in an extended characteristic map range for stabilizing the deflection. For example, the swirl in smaller-displacement engines may be increased under partial load and decreased under full load.
According to another advantageous embodiment, as mentioned, a squish gap may be present in the particular cylinder at top dead center of the piston, axially between the piston crown and the cylinder head; at top dead center of the piston, axially between the piston crown and the cylinder head, the squish gap has an axial height in a range of approximately 0.2% to approximately 3.0% of the piston diameter, preferably in a range of approximately 0.4% to approximately 1.2% of the piston diameter, and/or has a radial length in a range of approximately 6% to approximately 22% of the piston diameter, preferably in a range of approximately 9% to approximately 14% of the piston diameter. These dimensions of the squish gap have proven to be particularly advantageous for generating a squish gap flow which is suitable for generating the resultant flow.
In another advantageous embodiment, the piston step has a radial extension of 4% to approximately 20% of a piston diameter, and an axial height of approximately 2.5% to 7% of the piston diameter. This design of the piston step has proven to be particularly effective in forming the third combustion front.
In another advantageous embodiment, the injection holes of the injection nozzle for generating the injection jets have a ratio of the hole length to the hole diameter which is in a range of approximately 3 to approximately 11, and preferably in a range of approximately 4 to approximately 8.
In another advantageous embodiment, the injection holes have a rounding at an intake side facing away from the combustion chamber. By means of the rounding, in particular a rounding produced by hydraulic erosion, a flow increase in a range of 5% to 25%, preferably in a range of 10% to 20%, is achieved.
In another embodiment, the injection holes have a conical design and taper from the intake side to the exhaust side.
In one particular embodiment of the conical injection holes, a hole diameter at the intake side, optionally downstream from the rounding, is larger than a hole diameter at the exhaust side in a range of approximately 2% to approximately 25%, preferably in a range of approximately 5% to approximately 15%.
An elongated, relatively narrow, lobe-shaped injection jet having a high propagation rate may advantageously be generated by means of the conical injection holes, so that the injection jet spreads out only slightly in the combustion chamber, and the injection jet may thus strike the jet divider contour with high momentum.
In another advantageous embodiment, the piston is also equipped with a piston cone situated coaxially and centrally in the piston cavity. The piston cone advantageously has a cone angle that is smaller than a spray cone angle of the injection jets. For example, the cone angle may be approximately 124°±10°, in particular approximately 124°±5°. An apex of the piston cone through which the longitudinal center axis of the piston or the cylinder extends is axially offset toward the piston with respect to the piston crown and preferably also with respect to the piston step; i.e., the cone apex is axially recessed in the piston relative to the piston crown and in particular also relative to the piston step.
It is understood that the features mentioned above and to be explained below may be used not only in the particular stated combination, but also in other combinations or alone without departing from the scope of the present invention.